The catalogue consists of Table 2 (main data) and Table 3 (cross-index) which are available in electronic form at CDS (Strasbourg). Data on one multiple system are given as an example in Table 4 (click here) in the same format as in Table 2.
The form of Table 2 is the result of the compromise between two conflicting requirements. First, it seems natural to group various data on any given system in the same place to facilitate their retrieval, evaluation and comparison. On the other hand, the ASCII tabular file with fixed-format items is desirable for computer analysis. Table 2 is an ASCII file with lines of different types. Each line begins with the MSC code of the system based on the 1900.0 equatorial coordinates of the primary component. The MSC code is similar, but not equivalent to IDS (e.g. + and - signs are used instead of letters N and S). The second item of each line represents the key that defines the type of line and hence the information it contains. Systems are separated by blank lines. Using Unix command sort, it is easy to extract lines of the same type to a fixed format tabular file. Still the data on a system are not separated and can be searched and viewed by means of text editor or extracted by grep command if the MSC code is known. The MSC code can be obtained from other identifiers, such as HD or ADS, with the help of the cross-index file (Table 3).
The unknown parameters are given zero value. This may cause some inconvenience or mistakes. On the other hand, there are no missing items, and all lines of given type always contain equal number of items.
Table 4: MSC sample: data on one multiple system
The identification line is the first line for a system (code A) and contains common identifiers: HD number, DM number (BD, CD or CPD), HR number, ADS number (Aitken 1932), and other names if available (Flamsteed name, Gliese catalogue number Gl or GJ, variable star name, etc.). Zero means that the object is not found in a corresponding catalogue. Only one HD, HR or DM number corresponding to the primary is given for a system. The spaces in each catalogue identifier are converted to underline symbols.
The distance line (identified by the code D) contains additional information common to the whole multiple system. The equatorial coordinates for 2000.0 of the primary are given in the form of real numbers: hh.mmss and dd.''"". Then the proper motions in R.A. and declination in milliarcseconds per year are given. They were collected from various sources. Some proper motions were taken from the recent "4 million'' catalogue (Volchkov et al. 1992). Radial velocity is also provided, followed by parallax and its code as detailed in Sect. 3.3. The last item is multiplicity, marked by asterisk and a letter "T'' for trapezium systems.
Then follow the system lines, one per each sub-system,
with codes beginning by L. The numbers after L are the
hierarchical level of this sub-system (Sect. 2.4). System lines
contain the component designation, system type codes (Sect.
3.1), the decimal logarithm of the orbital period in days
(or asterisk if period is not known), angular
separation, its units (
for arcminutes, 
for
arcseconds and m for milliarcseconds),
position angle in degrees,
visual magnitude and
spectral type of the primary (or its B-V color index if the
spectral type is not known), visual magnitude and spectral
type or color index of the secondary, masses of the primary and
secondary components (in units of solar mass)
with by their codes. The last 20 characters of system line are
the remarks giving additional information either on the
system (identification in WDS or SB catalogues, short reference)
or on its secondary (HD, HR, DM numbers when available). In some cases
the orbital inclination is also given in the remarks.
When visual or spectroscopic orbital elements are known, they are
given on the two consecutive orbit lines which have codes
beginning with O and containing the same 4-number level
designations as the system lines. Visual and spectroscopic orbits are
given in the same format; in some cases they were combined together as
indicated by + sign in the remarks. The orbital elements are given
on the first orbit line in the following order: period, epoch of
periastron passage, eccentricity, semi-major axis with its units
(
for arcseconds and m for milliarcseconds), PA of
ascending node, longitude of periastron followed by letter A or B
specifying the component to which the last two parameters refer,
orbital inclination, semi-amplitudes of the primary and secondary,
center of mass velocity in km/s. Period and periastron epoch are
given either in years or in days (in this latter case 2400000 must be
added to the epoch). The second orbit line has letter r
appended to the code and contains remark that provides either
reference to catalogues of spectroscopic (SB) and visual (VBO) orbits
or bibliographic reference, given as journal abbreviation, volume,
page, year and sometimes author name. Codes T94 or T95 denote my
unpublished orbits.
The optional remark lines are coded by R and contain text in free format, mostly additional references.
Table 3 is a cross-index file that is provided for convenience. It has a fixed format. Each line contains MSC code of the multiple system, component identification letter and HD, DM, HR, ADS numbers. Missing numbers or items are replaced by asterisks. A multiple system may have several lines in Table 3 if one of its visual components has separate identifiers.
Figure 2:
Number of multiple systems N
within distance d
in the MSC. The expected 
proportionality is indicated by
the straight line. Distance limit of the Catalogue of nearby
stars is shown by the arrow
Figure 3: The relation between the logarithms of the orbital
periods of short- and long-period
sub-systems 
and 
(in days) at adjacent hierarchical levels
